IT is now almost a quarter of a century since Sylvia Plath killed herself, at the age of 30. Since her death, there have been several attempts to write her definitive biography, attempts that at some stage or another have run into trouble with those who safeguard the poet's literary estate: her husband, Ted Hughes, and his sister and agent, Olwyn Hughes. Whenever the ''truth about Sylvia Plath'' is discussed, you can be fairly sure that sooner or later the Hugheses will be mentioned with disfavor, as if the pair of them were somehow in the business of suppressing vital data. Nobody ever seems very clear about what kind of data this might be; the innuendoes persist, though, and it is hard to imagine what will put a stop to them.

On the face of it, the Hugheses can indeed be described as overcautious, not to say obstructive, in their dealings with those who wish to write Plath's life. It is known that there are Sylvia Plath writings under seal in libraries, not to be seen by anyone until the year 2013. It is also known that Ted Hughes destroyed a notebook that records Plath's most private thinking during the last months of her life (''because,'' he says, ''I did not want her children to have to read it''), and that another notebook from 1962 has simply, according to Mr. Hughes, ''disappeared.'' There is also the draft of what might have been Plath's second novel, ''Double Exposure''; that too is missing.

These disappearances, coupled with what one hears about the strict way the Hugheses have of granting permissions, can easily be made to sound suspicious, as if Ted Hughes were more interested in protecting himself, or some image of himself, than in letting his dead wife be heard. And since there are many admirers of Plath who have already cast Mr. Hughes in a villainous role - as the husband who dominated and then wronged her - such suspicions have gained ground over the years. There is a certain glamour in the projected notion of Plath as a woman posthumously victimized, just as (so it is thought by some) she was victimized in life.

It is difficult, with admirers of this sort, to plead for a more balanced view, and this new biography will not, I fear, make it any simpler. Linda Wagner-Martin is herself in favor of more balance, and she has learned enough about the more intimidating aspects of Sylvia Plath's personality to steer clear of any crudely feminist judgments of Mr. Hughes. Indeed, in her presentation, the marriage ended because Sylvia Plath expected too much from it. Even so, her book is prefaced with complaints about her dealings with the Plath estate. Mr. Hughes, she says, wanted some 15,000 words deleted from her manuscript. Although she made concessions, she did not go along with all of the suggested changes, and she has therefore been denied permission to quote at length from Plath's writings.

What were these changes, and why did they matter so much to Mr. Hughes (his list of objections, we are told, runs to 15 pages)? Were his objections self-interested, or did he straightforwardly believe that this biographer had got things wrong? But surely every biographer gets things wrong in the eyes of those who shared the life under discussion. What did Mr. Hughes see in this book that made him want to alter it so drastically? If the author had chosen to give us the details of her differences with him, some useful demystification might have been achieved. As it is, we go on wondering.

But wondering what? After all, it is not as if the world were starved of information on the life and private thoughts of this ferociously self-centered writer. We know about her glittering Smith College record, her precocious successes as a writer in her early 20's, her breakdowns and her first suicide attempt, her move to Cambridge, England, where she met and married Mr. Hughes - a poet far more self-assured and successful than she was. During the marriage, Plath did her best to be a ''good wife'' to a husband she adored, and her own literary ambition, which was huge, took - or pretended to take - second place to his. The explosive poems of ''Ariel'' were written after Mr. Hughes left her for another woman, and it is because of these poems that we have wanted to know more about her life. But what more can there be to know? We have in print 500 pages of her journals and 350 pages of ''Letters Home.'' We have dozens of memoirs and one or two full-length biographies. And, of course, we have the poetry, which in its later stages is scorchingly autobiographical. There are, to be sure, unpublished letters and large sections of Plath's journals that were not included in the edition co-edited by Mr. Hughes. But do these hold secrets that need to be revealed before we can be satisfied that we possess the truth about Sylvia Plath?

Ms. Wagner-Martin has been through all this material and is able to give us some facts we didn't have before. None of these seem wildly controversial, and none reflect all that badly on Mr. Hughes - nor, come to that, on Plath herself. Ms. Wagner-Martin's additions to the record are minor, circumstantial, and to savor them (or, sometimes, even to register that they are indeed additions) we really need to see her book alongside the published journals.

One effect of such an exercise is to make the Wagner-Martin text seem thin, impoverished. Even at her most hysterical, Plath has the intelligence and word power to defeat, or outshine, any paraphrase -and Ms. Wagner-Martin is not, in any case, the most subtle of reporters. Another effect is to make us wonder if the journals themselves could not have been less timidly prepared for publication.

For example, in the published edition, there are a number of declared omissions, material plucked from the middle of sentences and paragraphs and replaced by tantalizing dots. These censorings have, needless to say, given rise to uncharitable speculation. But whenever Ms. Wagner-Martin takes it upon herself to fill in these gaps, the result is invariably so tame as to make us wonder why the original omission was ever thought to have been worth the trouble. To take one instance, in the published journal we have this description of Plath's first meeting with Ted Hughes: ''Then he kissed me bang smash on the mouth (omission) . . . And when he kissed my neck I bit him long and hard on the cheek, and when we came out of the room, blood was running down his face. (Omission.) And I screamed in myself, thinking: oh, to give myself crashing, fighting, to you.''

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MS. WAGNER-MARTIN has been to the source, and she transcribes as follows:

''Then he kissed me bang smash on the mouth and ripped my hairband, off my red hairband scarf which has weathered the sun and much love, and whose like I shall never find again, and my favorite silver earrings: ha, I shall keep, he barked. And when he kissed my neck I bit him long and hard on the cheek, and when we came out of the room, blood was running down his face.''

We note that the second of the two original omissions is allowed to stand, so we must still remain ignorant of how, precisely, Mr. Hughes responded to that bite, but surely the material that is filled in need not have been excluded in the first place.

It is this sort of discussion that Ms. Wagner-Martin's book is most likely to provoke. As to Sylvia Plath, the picture we already have of her, and of the marriage, remains essentially unaltered, and Ted Hughes's own brief account of his wife's struggle to determine her ''real self'' still seems entirely plausible. One of the problems, though, with any biography of this poet is that Mr. Hughes himself has never given his version of the marriage. He has spoken abstractly about Plath's personality, and he has been a loving and scrupulous editor of her poetry, even though the best of it is fueled by an intense anger against him. He has permitted the publication of journals in which his own private conduct is laid bare in a way that he might well have found intolerable in a biography of which he was the subject. How much more discomfiture should be required of him? Plath in her last months was in a vengeful mood most of the time, and one can well imagine that there are unpublished writings that, if released, would seem to invite some sort of line-by-line rebuttal by Mr. Hughes. Temperamentally, he is as reticent as Plath was effusive, and that reticence surely deserves more respect than it is usually granted.

And yet does one really want more paraphrase of the prohibited material, more guesswork, more sentences like this one (from Ms. Wagner-Martin): ''They spent the night at his second-floor flat on Rugby Street in London, reciting poetry, making love, finding their alter ego in each other - rebellious and isolated, strong and erotic and gifted''? Ms. Wagner-Martin's style veers between colorful empathizing and businesslike terseness (''Summer was slow, relaxed and - for Ted - productive. Everything he wrote was good''), and her book is somewhat unbalanced by her wish to exhibit the fruits of her considerable research. That is to say, if an event is covered by the published journals, she will tend to skimp it, but if it is ''new,'' it gets more emphasis than it necessarily deserves. This again brings us back to the problem of permissions, and to the thought that the next Sylvia Plath biography will surely have to be one that enjoys the full cooperation of Mr. Hughes. In the meantime, there is a question that is too rarely pondered: what would Sylvia Plath have wished to preserve, if she had lived? DO NOT OPEN UNTIL 2013

It's a five-hour drive from Michigan State University in East Lansing to Indiana University in Bloomington. Linda Wagner-Martin, a professor of English at Michigan State, has made the trip half a dozen times since 1977, when Indiana obtained a collection of Sylvia Plath's papers.

At first, it was academic fascination that led Ms. Wagner-Martin to drive past cornfields to poke through the 3,324 items, ranging from school yearbooks to unpublished poems. ''But as I began looking at the material, it became clear there weren't many scholars using that collection,'' she said in a telephone interview from her office. After Smith College in Northampton, Mass., got other Plath papers in 1981, she decided it was time for a book drawing on both collections.

Ms. Wagner-Martin, who gathered information from more than 200 of Plath's friends and acquaintances, said she ran into trouble with Plath's husband, Ted Hughes, and his sister, Olwyn Hughes. ''They gave meticulous lists of what they wanted deleted. But they didn't give many reasons. A lot of the stuff they wanted out was from academic books and interviews already done with Ted Hughes.''

Some of Plath's papers have been sealed until 2013 and others until the deaths of her mother and younger brother, but Ms. Wagner-Martin said there was ''ample material'' available. ''I'd give a lot to have those sealed papers,'' she said. ''But, as my students say, I'll have to wait until 2013 to find out how wrong I was.''

STEWART KELLERMAN

Ian Hamilton is the author of ''Robert Lowell: A Biography.''

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A version of this review appears in print on October 25, 1987, on Page 7007012 of the National edition with the headline: THE TATTY WRECKAGE OF HER LIFE. Today's Paper|Subscribe